"The estuary doesn’t yield all its secrets on first glance. An hour or so out from the Isle of Sheppey, we arrive at seven bizarre constructions that look as if they belong in War of the Worlds..."

An image of the Maunsell Red Sands forts from William Raban’s "Thames Film".

The smell comes first. The sea salts the air, and the mud over which it ebbs and flows adds a rotting, brackish note. Newcomers glance around, wafting their hands suggestively before their noses, keeping it out. To savour a proper lungful of this air is to admit something difficult - that this unarguably ugly landscape, where the grey sea meets the grey sky with barely a smudge of mud in between, is beautiful.

There is more sky here, where the Thames and the Medway meet the North Sea, somehow. The great man-made structures that thrust upwards into it, like the power stations on the Isle of Grain or the cranes at the port of Sheerness, only serve to emphasise how much more there is. What would be an eyesore elsewhere is accepted by the people who sail these waters without comment, as they accept the smell or the mud.

The estuary doesn’t yield all its secrets on first glance. An hour or so out from the Isle of Sheppey, we arrive at seven bizarre constructions that look as if they belong in War of the Worlds - the Maunsell Red Sands forts. Built in 1943, these now-rusty steel boxes-on-legs housed hundreds of men during WWII who used anti-aircraft guns to bring down planes on their way to bomb London. Planting these forts miles out to sea was no mean feat of engineering, and they are a reminder of how desperate, and improbable, some of our war-time defences were. Since being decommissioned in the 1950s, the forts have housed pirate radio stations, trespassers, film crews, scientists and conservationists, but no fixed plan has ever been made for their future. Decades later, the sea is wearing them down - it’s not clear how much longer they can stand and wait for us.

William Raban’s 1987 work Thames Film uses a clever technique of sliding contemporary and historical footage of the estuary together so that different moments in time appear to co-exist. Watching the forts recede over the horizon again, it seems to me as if the whole estuary is made up of such layers. As a child, I spent my weekends and school holidays staring at the horizon from the rolling deck of my parents' boat, curious as to why, if you could sail anywhere, it would be on this smelly, featureless stretch of water. Once a teenager, dragged unwillingly on night-time excursions to the Netherlands, I would sulk on the foredeck, dropping angry tears into the miraculous phosphorescence that bloomed under the boat's bow. Returning now after an absence of years, I want to be able to recapture that grim fury, but it won't come. I can only stare at the sky.